Hydroxy functional polydiene polymers are well known. It has been shown that formulations containing these polymers, a melamine resin, and an acid catalyst can be cured by baking under normal bake conditions. Most of these polymers are homopolymers of one diene or another. For example, monohydroxylated polybutadienes are known in the art for use in adhesive formulations. U.S. Pat. No. 4,242,468 describes solventless polyurethane coatings having improved flexibility resulting from incorporation of monohydroxylated polybutadienes. Epoxidized versions of hydroxylated polybutadienes are known as well. Low viscosity epoxidized polydiene polymers are also known, especially for use in adhesives. Such polymers are described in commonly assigned U.S. Pat. Nos. 5,229,464 and 5,247,026.
Compositions based on functional polydiene polymers cured with a crosslinking agent are usually fairly soft and, although they can be very useful in some applications, have limited adhesion. Attempts to increase hardness and adhesion by increasing crosslink density with a reinforcing agent, such as a low molecular weight diol or triol, and higher levels of crosslinking agent were unsuccessful because the reinforcers and crosslinkers are relatively polar and so are incompatible with the relatively nonpolar polydiene polymers. Incompatibility of the components leads to poor properties, such as gloss, in the cured composition or, even worse, the compositions may phase separate upon standing. U.S. Pat. No. 5,500,481 provides a solution to this problem.
The above-referenced patent shows that the compositions described therein adhere well to steel substrates. Data in this application show that the compositions of the above-referenced patent adhere well to thermoplastic polyolefin (TPO) surfaces. However, the data show that not all of the compositions which adhere well to TPO will also adhere to steel. It would be advantageous to have a way to utilize these compositions on steel substrates because, for example, in an end use area such as automotive, the same compositions could be used to coat both the TPO surfaces of the car and its steel surfaces as well. The present invention provides such a method.